Acid and alkaline treatments for enhancing the growth of rhizobia in sludge

Abstract

Wastewater sludges have been proposed as an effective media for the production of rhizobia. The effect of total suspended solid (TSS) concentrations and pretreatments of sludge on the growth of Sinorhizobium meliloti were investigated. Acid (pH 2.0–6.0 obtained with H2SO4) and alkaline (50–200 mequiv.wt./L of NaOH) treatments were applied to enhance the biodegradability of primary (0.325%–3.2% TSS obtained by dilution of original sample) and secondary (0.2%–0.4% TSS obtained by concentration of original sample) sludges. In primary sludge without pretreatment, the highest cell count (11.10 × 109 cfu/mL) was obtained with 1.3% TSS. However, a maximum cell count of 13.00 × 109 cfu/mL was reached using an acid treatment of pH 2.0 and a 0.325% TSS concentration. Moreover, the alkaline treatment with 100 mequiv.wt./L of NaOH and 0.65% TSS increased the cell yield to 21.00 × 109 cfu/mL. For secondary sludge without pretreatment, no enhancement of growth was observed while increasing TSS concentration. This may be due to the increase of inhibitory substances, such as heavy metals, and of the Ca and Mg concentrations. As in primary sludge, some acid and alkaline treatments of secondary sludge tend to improve the cell count of S. meliloti. However, the highest value of 9.80 × 109 cfu/mL obtained with 0.4% TSS at pH 2.0 was lower than that obtained with primary sludge. It was also observed that S. meliloti grown in treated sludges maintained its capacity to nodulate alfalfa.Key words: wastewater sludge, rhizobium, inoculum, sludge pretreatment, nodulation.